There may be many sources in a motor vehicle that generate and broadcast time varying signals. For example, motor vehicles frequently employ switching power supplies that utilize pulse width modulation to convert the voltage from the vehicle power supply (usually twelve volts) to whatever particular voltage levels are required to run the various electronic modules connected to the power supplies. The switching power supplies nominally operate at an operating frequency in the frequency range from about 20 to 500 kilohertz (kHz). The operating frequencies or harmonics of the operating frequencies generated by the power supplies may cause interference with some motor vehicle radios, especially with the amplitude modulation (AM) frequency band of a motor vehicle radio. The AM frequency band extends from 520 kHz to 1710 kHz and especially the low-order harmonics of switching power supply operating frequencies may fall within that AM band frequency range. Because AM radio tuners have a ten kHz resolution, radio interference is experienced whenever a power supply operating frequency or harmonic of that operating frequency is within plus or minus five kHz (the interference range) of the frequency to which the radio is tuned.
As disclosed in commonly assigned patent application Ser. No. 10/665,795, the entire disclosure of which is incorporated herein by reference, the problem of interference from switching power supplies and other sources of time varying signals in a motor vehicle can be largely overcome by switching the operating frequency of potential sources of such time varying signals to insure that the operating frequencies and all of the harmonics of the operating frequency are outside the interference range of the frequency selected by the tuned radio. The method disclosed in the above identified patent application comprises the steps of communicating to the source of the time varying signal the frequency selected by the tuned radio, comparing the selected frequency to the operating frequency of the source of the time varying signal and to the harmonics of that operating frequency, and adjusting the operating frequency if the operating frequency or any of the harmonics of the operating frequency fall within a predetermined interference range of the selected frequency. A further problem occurs, however, when using such frequency shift interference reduction in a motor vehicle having a motor vehicle radio that is capable of executing a seek/scan function. The message used to communicate the selected frequency must be sent out over a bus that couples the radio and the module that is the source of the time varying signal. Unfortunately, sending such a message can cause a problem because of bus latency. The receiving module must receive the message, decode the message, and then adjust its frequency (if necessary). If these steps are carried out at every tuned frequency encountered during the seek/scan function, including frequencies corresponding, not to a radio station signals, but to the operating frequency or a harmonic of the operating frequency of the module generating the time varying signal, this will result in a significant delay and a correspondingly long seek/scan time.
Accordingly, it is desirable to provide a method for suppressing motor vehicle radio interference, caused by a module within the motor vehicle that generates a time varying signal, when the motor vehicle radio is capable of executing a seek/scan function. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.